This caterpillar like creature is the larva of a sawfly, probably of the genus Dolerus. Its mother will have used her saw-like ovipositor to cut a slit in the horsetail in which to lay eggs. Once hatched, the larva is perfectly formed for chowing down horsetail fronds, while remaining largely unnoticed.
|Dolerus Spp. - Horsetail Sawfly|